own automatically defined data type in Python

Question

Let's say I have following code:

mystring='abcd'
print(type(mystring))

Python understands that mystring is a string object and returns <class 'str'>. How does this happen? What if I want to declare my own data type that supports the same logic, that is when I type print(type(mystring)) I receive <class 'myClass'>, not python default class <class 'str'>. How can I achieve that?

Update: I am interested in how to rearrange the logic of class auto-definition? I don't want to declare variable explicitly as an object of myClass by myString = myClass('abcd'). I just want to type myString = 'abcd' and be sure that Python has defined myString as instance of myClass. Is that possible? May be it is all about inheritance/overriding str class?

What do you mean, how does this happen? You have a string and you got its type, and its type is str, what else would happen? — kaya3
– kaya3, Commented Jun 13, 2021 at 16:52
To define a custom type you define a class. Indeed, "class" and "type" in python are synonymous. Indeed, type is just a class, the class of all classes, i.e. a metaclass if you want to know a little more about how it all works — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jun 13, 2021 at 18:14
Beside the point, but class names should be UpperCamelCase, so, MyClass. — wjandrea
– wjandrea, Commented Jun 13, 2021 at 18:17
'abcd' is always in instance of str, you can't make it something else without being explicit about it. — interjay
– interjay, Commented Jun 13, 2021 at 18:17
For reference: Lexical analysis > Literals > String and Bytes literals — wjandrea
– wjandrea, Commented Jun 13, 2021 at 18:20

wjandrea · Accepted Answer · 2021-06-13 18:34:05Z

1

You can't do what you want to do. How literals are defined in Python is inherent to the language. You can definitely override str which will change what happens when it's actually used, but this isn't recommended.

If you could globally overwrite the inherent primitive types of the language how would you expect other packages to behave?

>>> class MyClass:
...     def __init__(self, string):
...         self.strlen = len(string)
...

>>> str = MyClass

>>> type("foo")
str

>>> type(MyClass("foo"))
__main__.MyClass

>>> type(str("foo"))
__main__.MyClass

edited Jun 13, 2021 at 18:34

wjandrea

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answered Jun 13, 2021 at 18:22

blueteeth

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Comments

Paras Gupta · Accepted Answer · 2021-06-13 16:56:23Z

0

You are talking about Object Oriented Programming in python. You can make your own custom class with specific functions.

For example:

class myClass(str):
    pass

myString = myClass('abcd')
print(type(myString))

Output:

<class '__main__.myClass'>

answered Jun 13, 2021 at 16:56

Paras Gupta

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3 Comments

KiNest Over a year ago

Thank you for your answer but my question concerns a little bit different issue: I am interested in how to rearrange the logic of class auto-definition? I don't want to declare variable explicitly as an object of myClass by myString = myClass('abcd'). I just want to type myString = 'abcd' and be sure that Python has defined myString as instance of myClass. Is that possible? May be it is all about inheritance/overrideinf str class?

juanpa.arrivillaga Over a year ago

@KiNest what you are asking for doesn't make any sense. Python doesn't have "class auto-definition". You create objects in Python, and any object has a type. myString = myClass('abcd') is not a declaration, python doesn't have variable declarations.

juanpa.arrivillaga Over a year ago

@KiNest and note, you probably almost certainly do not want to inherit from str. Maybe, but I suspect not

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